DESCRIPTION (Verbatim from the Applicant): Parathyroid hormone (PTH) promises to be a useful anabolic agent for bone. However, the mechanism by which PTH stimulates bone formation remains imperfectly understood. We propose that at least part of the means by which PTH stimulates bone formation is through other growth factors, in particular insulin like growth factor 1 (IGF-1). IGF-1 plays an important role in bone formation, and modulates the ability of PTH to do likewise. The availability of an animal model in which IGF-1 production has been eliminated by homologous recombination provides an opportunity to test the degree to which IGF-1 mediates and/or regulates the skeletal response to PTH. The hypothesis we propose to test is: IGF-1 mediates the anabolic action of PTH on bone by stimulating osteoprogenitor cell proliferation and inhibiting osteoblast apoptosis. We will test this hypothesis by accomplishing three Specific Aims. 1. Determine the role of IGF-1 in regulating the in vivo response of the mouse skeleton to the intermittent administration of PTH. This will be performed by analyzing the ability of PTH to stimulate bone formation in IGF-1 deficient mice as assessed by fat free weight and calcium content of bone, bone histomorphometry to quantitate cell numbers and bone formation rates, bone microstructure using micro-CT, location and identity of proliferating bone cells by Bride labeling, localization and quantitation of the mRNA and protein levels of IGF-1, the receptors for PTH and IGF-1, and the markers of bone formation, and localization and quantitation of apoptotic cells. 2. Determine the role of IGF-1 in regulating the in vitro response of bone cells to PTH. This will be performed using bone marrow stromal cells and osteoblasts obtained following in vivo administration of PTH or studied in vitro in the presence of PTH. The response of these cells to PTH with respect to markers of proliferation, differentiation, and apoptosis will be assessed. In the event IGF-1 is found to mediate the PTH response, the roles of the MAPK and P13K pathways in this process will be explored. 3. Determine whether transgenic mice with targeted expression of IGF-1 in their osteoblasts can restore PTH responsiveness to the bones of IGF-1 deficient mice. To accomplish this aim mice overexpressing the IGF-1 gene in osteoblasts either under the osteocalcin or the alpha1(1) procollagen promoter will be bred with the IGF-1 deficient mouse and the response to PTH evaluated using the assays developed in the preceding Specific Aims. The results of these experiments should determine the degree to which PTH is dependent on IGF-1 for producing its anabolic actions on bone, and initiate the exploration into the mechanisms by which this occurs.